Strong to weak coupling transition in low misorientation angle thin filmYBa2Cu3O7−xbicrystals

Abstract

Detailed transport measurements were made of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ thin-film [001] tilt bicrystals with misorientation angles of 3°, 5°, 7°, 10°, 15°, and 20°, encompassing the angular regime where the transition from strong to weak coupling occurs. The study includes measurements of intragrain and intergrain extended voltage-current characteristics in applied magnetic fields that range from zero to well above the irreversibility field. The results show that the strong-to-weak coupling transition is progressive at 77 K, occurring at misorientation angles between 7° and 10° in zero field, and between 10° and 15° in higher magnetic fields. The shapes of the voltage-current characteristics of the 7° [001] bicrystals and the ratio of the inter- and intragranular critical current densities are particularly sensitive to individual sample preparation conditions, suggesting that substrate-film interdiffusion along the grain-boundary dislocations is controlling the effective size of the superconducting channels between the dislocation cores. The linear decline of the intergranular critical current density with misorientation angle predicted from present dislocation core overlap models is not found, showing that additional features of the grain-boundary nanostructure and the mechanism of intergranular current flow need to be invoked in order to explain transport across low-angle ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ grain boundaries.